I moved into my house about five years ago and was told that it had a waterproofed basement. The inspection didn’t note any issues with the basement, including the waterproofing system.
After moving in, we noticed that some areas of the basement had what looked to us at the time like a waterline, where it was black about 6 inches to one foot up the wall. We thought that water must have gotten in at some point (weird) and tried to scrub away as much as we could with a bleach based cleaner.
Fast forward five years later to today, when we noticed that it had started to return in some areas, primarily where air is more obstructed (behind a shelf, behind the washer, etc). This time, it more clearly seemed like it was coming up from the ground somehow.
Here’s a pic of what we’re dealing with:
I’m at a loss as to what to do. Is this harmful? Should I attempt to clean it myself? Should the waterproofing system be preventing it? Where could it even be coming from?
FWIW, we have a dehumidifier in the affected space, and a vent fan. So it does get humid down there, but I thought we were kind of already doing the things we could do…